This invention relates to the production of flexible polyurethane foams by a process involving the reaction of tolylene diisocyanate with a polyether polyol in the presence of a small amount of water and a catalytic amount of a polymerization catalyst. More particularly, the invention provides an improved method for flame retarding such polyurethane foams by incorporating dibromoneopentyl glycol into the foam matrix without downgrading the flexibility or porosity of the foam.
Flexible polyurethane foams made from tolylene diisocyanate and a polyether polyol comprise a very large segment of the polyurethane foam industry and are extensively used in fabricating automobile and furniture upholstery. Because of the inherent combustibility of polyurethane foams, which generally burn uncontrollably after ignition, there has been an extensive effort in the polyurethane foam industry during the past several years to produce flame retardant foams. These efforts have been stimulated by recent governmental regulations requiring improved safety standards for polyurethane foams used in automobiles, one of the most important of which regulations is Motor Vehicle Safety Standard Docket 302, published in the Federal Register on Jan. 8, 1971 and effective Sept. 1, 1972.
Flame-proofing flexible polyurethane foams, which have an open-cell porous structure, is much more difficult than flame-proofing closed-cell rigid foams, since open-cell structures permit the flame to obtain oxygen through the back face of the porous structure and also permit any volatilizable flame retardant to escape ahead of the flame front.
Many different methods have been suggested and commercially tested for flame retarding flexible polyurethane foams, all of which methods involve the addition to the foam reactants of some flame retardant prior to or during the polymerization reaction so that the flame retardant is uniformly distributed throughout the resultant foam. For convenience, these flame retardants may be characterized as either (1) non-reactive inorganic compounds which remain as discreet solid particles after dispersion through the foam matrix, (2) non-reactive organic compounds which remain dissolved in or dispersed throughout the foam matrix but are not chemically bound to or as part of the foam matrix, and (3) reactive organic compounds which enter into the polymerization reaction and become chemically bound as part of the foam matrix. Each of these methods presents difficulties.
The most common example of the first class of flame retardants, namely the non-reactive inorganic compounds which remain as discreet solid particles after dispersion throughout the foam matrix, is illustrated by the use of antimony trioxide and zinc oxide. As shown in U.S. Pat. No. 3,574,149, flame retardant flexible polyurethane foams may be obtained by dispersing in the foam reactants a mixture of antimony trioxide and zinc oxide in polyvinyl chloride. Insofar as these solid flame retardants are not compatible with the polyurethane foam matrix, flame retardancy is reduced and may be lost during washing and/or humid ageing of the foam. Moreover, since the required use level of these additives ranges from 20 to 30 percent by weight of the total weight of the foam reactants, this system always changes the foam properties. Finally, a high bun exotherm (in which the interior temperature of the polyurethane foam bun reaches temperatures in excess of 150.degree.C.) results in a scorching of the interior of the foam due to the presence of any thermally unstable polyvinyl chloride.
The second class of flame retardants used in flameproofing flexible polyurethane foams is the non-reactive organic additive which acts as a plasticizer when dissolved in or dispersed throughout the foam matrix. Examples of such plasticizers include the halogenated phosphonate-phosphite sold by Monsanto Company under the trademark Phosgard C-22R and described in U.S. Pat. No. 3,014,956, as well as 2,2-bis-(chloromethyl)trimethylene-bis-[di-(2-chloroethyl)phosphate] sold by Monsanto Company under the trademark Phosgard 2XC-20 and described in U.S. Pat. No. 3,192,242. Another example is tris-(2,3-dibromopropyl)phosphate which is sold by Michigan Chemical Company under the tradename LV-T23P. The use of halogenated phosphate esters in flexible foam for flame retardance results in mild to severe loss of foam properties, which is dependent on the use level of additive, and in mild to severe foam discoloration, which is dependent upon interior bun temperatures, with temperatures in excess of 150.degree.C. often resulting in very severe darkening of the center of slabstock bun. On ageing, flame retardancy is often lost due to the small but finite vapor pressure of the halogenated phosphate plasticizer.
The third class of flame retardants suggested for flame-proofing polyurethane foams are those reactive organic compounds which enter into the polymerization reaction and become chemically bound as part of the foam matrix. Examples of such flame retardants may be found in the article by Parrish et al. J. Cellular Plastics, 5, 348-57 (1969) and in U.S. Pat. Nos. 3,257,337 and 3,597,503. Insofar as such reactive organic compounds contain halogen, their efficacy as flame retardants in polyurethane foams is related to the relative ease with which the resultant foam forms hydrogen chloride and/or hydrogen bromide in the flame. These non-burning gases are believed to interfere with the free-radical mechanism of the burning reaction and stabilize the system against free-radical decomposition of the polyether chains in the foam matrix.
The current trend in flame-proofing flexible polyurethane foams has been to seek methods of employing a copolymerizable flame retardant notwithstanding the fact that the past history of using such reactive flame retardants has been mostly of theoretical rather than practical value. The reason for this is that foam processing with reactive flame retardants has been so difficult that it has prevented the adoption of these flame retardants on a commercial scale. However, the preparation of small scale laboratory foams have shown that reactive flame retardants may be extremely efficient in preventing flame propagation and do not suffer from loss of effectiveness on long-term ageing. The major shortcoming to this approach is the difficulty experienced in producing flame retardant polyurethane foams on a commercial production basis.
Polybrominated polyols are known to impart flame retardancy to the polyurethane chains when copolymerized with the other reactants which form the urethane linkage. One such polybrominated polyol is dibromoneopentyl glycol ("DBNG"), the Geneva name for which is 2,2-bis-(bromomethyl)-1,3-propanediol. An early example of the use of dibromoneopentyl glycol to flame-proof polyurethane coating compositions (which are not foams) may be found in U.S. Pat. No. 3,542,740.
Although dibromoneopentyl glycol has been used as part of a complex ternary flame retardant system (containing antimony trioxide and a chlorinated hydrocarbon polymer) in flexible polyurethane foams made from tolylene diisocyanate and polymethylene polyphenylisocyanate interpolymerized with polyether polyols (as shown in U.S. Pat. No. 3,738,953), this copolymerizable flame retardant has never been used successfully as the only flame retardant in flexible polyurethane foams made from tolylene diisocyanate and polyether polyols because it has been impossible to incorporate dibromoneopentyl glycol in such urethane foam systems without obtaining closed or tight cell structures which, while suitable for flame retardance per se, make the resultant foam unsuitable for many uses.